1. Field of the Invention
This invention relates to high resilience polyurethane foams, and more particularly to the use of minor amounts of certain polyester polyols to improve the resilience of flexible polyurethane foams made from polyether polyols. The high resilience polyurethane foams made according to this invention are obtained directly, without the necessity of mechanical crushing of the cured foam. The method disclosed is especially applicable to the production of free-rise slabstock, but it can also be employed to produce high resilience foam in closed molds, if desired.
2. Description of the Prior Art
Flexible polyurethane foams prepared by the reaction of polyether polyols and organic polyisocyanates in the presence of blowing agents are well known, and have been made commercially in various forms including molded shapes, sheets, and blocks of circular and rectangular cross-section. Such products were found to be of use in a number of applications such as insulation and packaging, but were not completely satisfactory in cushioning applications due to lack of sufficient resilience. Typical uses requiring resiliency include cushions, mattresses, automotive and furniture upholstery, and shoe inner soles.
The deficiency in resilience of polyether polyurethane foams is believed to be due to a number of factors, including the molecular composition of the polyurethane polymer and the fact that the foams contained too high a proportion of closed cells. A great many attempts have been made to overcome the deficiency in resilience. While these have provided some improvements, none has been completely successful and many add unduly to cost by way of more expensive raw materials or additional processing steps.
One of the known means for improving resilience is to subject a foam having a high proportion of closed cells to mechanical crushing, as between a pair of rolls for example. This method causes some of the closed cells to rupture, thus increasing the degree of openness and rendering the structure less tight and more resilient. Although crushing has beeen widely used in the foam industry, it does not improve resiliency to the desired degree and necessitates additional equipment and processing with a consequent increase in cost.
Another approach to improving resiliency is the use of the so-called polymer-polyols such as those described by Patten et al in Journal of Cellular Plastics, November/December 1974, page 276 et seq, as partial or total replacements for conventional polyether polyols. These polymer-polyols are produced by the in situ polymerization of one or more vinyl monomers, such as styrene and acrylonitrile, in the presence of a conventional polyether polyol. Although these materials may provide improved resiliency in foams made under ideal conditions, they have been found to be deficient in processability when used in free-rise slab stock formulations under actual plant conditions.
Various other means for improving resiliency have been disclosed in prior patents. Examples are: the use of an undistilled tolylene diamine phosgenation product having a controlled acidity, disclosed in U.S. Pat. No. 3,801,518 to Irwin et al; the use of a mixture of alkanols of octahydrothieno (3,4-b) pyrazine 6,6-dioxide as chain extenders, as disclosed in U.S. Pat. No. 3,821,132 to Mao et al; the use of polyether polyols capped with urethane or urea end groups, as disclosed by Fabris et al in U.S. Pat. No. 3,823,096; the use of isocyanurate polyols as curing agents for molded foams, as disclosed by Taub in U.S. Pat. No. 3,856,718; the use of a mixture of polyols comprising an ethylene oxide tipped polyester polyol and a large proportion of a second polyester polyol containing at least 40% by weight of oxyethylene groups at least some of which are in non-terminal positions, as disclosed by Fishbein et al in U.S. Pat. No. 3,857,800; and the addition to the reaction mixture of a solid polymer of ethylenically unsaturated monomers free from groups reactive with NCO or OH groups, as disclosed by Blankenship in U.S. Pat. No. 3,869,413.